The objective of this project is to obtain validation of the efficacy of K-Mag (IND 32,284) demonstrated from one recently completed randomized trial, by conducting a multiclinic randomized trial. Participating patients will be those with recurrent calcium nephrolithiasis who have recently undergone Extracorporeal Shock Wave Lithotripsy for removal of stones. It will test the hypothesis that K-Mag will be more effective than placebo in inhibiting stone formation. the treatment outcome will be analyzed by survival probability analysis (percentage of patients remaining stone-free.